The synthesis of highly functionalised dienes for natural product synthesis

The Lewis acid and Bronsted acid mediated intramolecular cyclisation reactions of allenyl epoxides to 1,3-dienes are reported. Titanium(IV) chloride and hydrofluoric acid give six membered carbocyclic dienes, whereas stannic chloride affords a 2:1 mixture of six and seven membered rings respectively. Diethylaluminum chloride mediated cyclisation furnishes exclusively a seven membered ring 1,3-diene. Trimethyloxonium tetrafluoroborate gives a six membered permethylated diene.